1. Field of the Invention
The present invention relates generally to receivers for TVRO earth stations capable of receiving audio and video signals from a plurality of orbiting earth satellites. More particularly, this invention relates to a TVRO receiver system which includes a tracking filter arrangement capable of adequately rejecting image frequencies generated by received signals.
2. Description of Related Art
In satellite communication systems, orbiting satellites are used to receive modulated carriers transmitted in the form of electromagnetic waves from a transmitting earth station. The satellites retransmit the incident electromagnetic waves, after they have been electronically reformatted in some fashion, to receiving earth stations. The earth stations in such systems generally comprise transmitting and/or receiving power stations which function in conjunction with an antenna subsystem. TVRO earth stations in particular are adapted to receive such satellite broadcast signals and generally comprise a receiving antenna such as a paraboloidal dish, a low noise block converter located at an outer antenna site, and a superheterodyne receiver located in the vicinity of a television set adapted to display received signals.
Satellite signals received by the antenna are first converted to a lower IF frequency. The conversion is generally effected by a down converter which transforms a single selected channel to a first IF frequency, or a block converter which converts all channels possessing a common polarity to a first IF block of frequencies typically ranging from 950 to 1450 MHz. The down-converted block of frequencies is passed to the receiver end through a coaxial cable and the receiver converts the first IF signals to a second IF frequency range which traditionally has been centered at about 70 MHz in most TVRO systems. State-of-the-art TVRO receivers are increasingly using higher second IF frequencies, e.g., 612 MHz, in order to ensure that frequencies provided by the local oscillator stage are above the first IF block of frequencies and thereby prevent local oscillator frequencies from interfering with received signals.
Regardless of the actual second IF frequency being used in a TVRO system, a basic problem associated with the reception of satellite signals in such systems is the existence of image frequencies generated as a function of the local VCO frequency and the desired IF frequency. More specifically, in TVRO receivers, the second IF frequency represents the preselected constant difference frequency between the first IF center frequency and the corresponding VCO output frequency for each transponder signal. In effect, the VCO output frequencies are regulated in such a way that the second IF frequency is centered on the corresponding first IF center frequency for signals received from each transponder. For example, a VCO output frequency of 1430 MHz causes the 612-MHz second IF to be centered on the 818 MHz first IF center frequency for signals being received from a transponder. As a result, signals corresponding to the 818 MHz IF frequency are selected for reception. However, an image frequency corresponding to the sum of the VCO frequency and the desired IF frequency is also generated and constitutes noise which can severely distort the desired signals unless rejected by the receiver in some fashion. In the above example for instance, an image frequency is generated at 2042 MHz sum of the VCO frequency, i.e., 1430 MHz, and the desired IF frequency, i.e., 612 MHz). Accordingly, it is important that this image frequency of 2042 MHz be rejected in order that the desired signal at 818 MHz be adequately received.
It has been conventional to achieve the rejection of image frequencies by the use of bandpass filters. Such filters are designed to provide a high degree of rejection at the image frequencies. The use of a bandpass filter to achieve attenuation of image frequencies entails a number of disadvantages arising from the tilt or slope in the response of the bandpass filter unless it is accurately and exactly tuned onto the received frequency. The quality factor of the bandpass filter has to be substantially high in order to achieve a given amount of rejection at the image frequency. Another problem peculiar to the reception of satellite signals which generally include wide-band FM signals, is that the edges of the modulation bandwidth can vary substantially; accordingly, the bandpass filter must be tracked in such a manner that it is tuned to the center of the desired frequency band. This can only be achieved by accurate tuning and matching of the variable capacitance diodes used to define the changing passband of the bandpass filter. As a result, bandpass filters for frequency rejection in TVRO receivers are expensive since the need for precisely matched and accurately tunable varactor diodes increases both component cost and production time.
This problem is further compounded when a TVRO system is to be designed for operation with significantly different second IF frequencies. More specifically, the second IF frequencies typically used in Europe are substantially lower than the 612 MHz IF being used in other parts of the world. For instance, a commonly used IF frequency in Germany is 480 MHz. In order for a bandpass filter to function effectively in rejecting image frequencies generated by use of both the 612-MHZ and the 480-MHz IF frequencies, the quality factor of the bandpass filter must be extremely high, and the tracking and tuning of the filter components also must be correspondingly more accurate. In combination with the need for precisely matched varactor diodes, this renders the use of bandpass filters for rejection of image frequencies at different, high, IF frequencies a problematic and expensive approach.